An electrochemical system is a system that either derives electrical energy from chemical reactions, or facilitates chemical reactions through the introduction of electrical energy. An electrochemical system generally includes a cathode, an anode, and an electrolyte, and is typically complex with multiple heterogeneous subsystems, multiple scales from nanometers to meters. Examples of these systems include fuel cells, batteries, and electroplating systems. On-line characterization of batteries or fuel cells in vehicles is difficult, due to very rough noisy environments.
On-line characterization of such electrochemical systems is desirable in many applications, which include real-time evaluation of in-flight batteries on a satellite or aviation vehicle, and dynamic diagnostics of traction batteries for electric and hybrid-electric vehicles. In many battery-powered systems, the efficiency of batteries can be greatly enhanced by intelligent management of the electrochemical energy storage system. Management is only possible with proper diagnosis of the battery states.
Batteries based on lithium (Li), such as lithium-ion batteries, are attractive due to their high energy density compared to other commercial batteries. Lithium-ion batteries are used commercially in computers, cell phones, and related devices. Battery lifetime is often a critical factor in the marketplace, especially for commercial, military, and aerospace applications. Battery life is often the limiting factor in many aerospace products, such as satellites. Unfortunately, lithium-ion batteries have caused fires in cars, computers, mobile devices, and aircraft. Thus, there is a safety need to dynamically monitor the battery states.
Thus in many lithium-ion battery applications it is preferred to periodically or continuously monitor the electrochemical potential of the electrode materials in the battery as a measure of their state of charge or state of health. Knowledge of the state of charge or state of health may be important for high charge rate or high discharge rate applications such as power tools and partially or fully electrified vehicles. The electrochemical potential of electrode materials may be altered and permanently lost if, for example, the cells of the battery are discharged too rapidly or overcharged often causing lithium to plate on the surface of the negative electrode.
In order to monitor the electrode materials, a reference electrode has been placed in one or more cells of the battery in such a way as to monitor the state of charge of at least one or both of the positive and/or negative electrodes of the cell. The connection is a high-impedance connection that draws very little current from the positive or negative electrode, but the potential (voltage) of the positive and/or negative electrode in the cell electrolyte with respect to the reference is measured. These voltage values (reference electrode vs. positive and/or negative electrode) may be obtained in a battery as the cells are being charged or discharged, and collected for computer analysis and control of the discharge and charge rates of a battery.
Conventional reference electrodes are often inserted between the positive and the negative electrodes and their area is usually kept small in order to minimize the “shielding effect” that distorts the current path between the positive and negative electrodes. However, a small electrode area can lead to large polarization resistance that can potentially give inaccurate potential readings. Other conventional methods involve placing the reference electrode surrounding the battery outside of the direct current path between the positive and negative electrodes. The drawbacks of these designs are that the system measures the potential of electrode edges which rarely represent the true potential of the electrode. Such distortion is further exaggerated at high current densities.
For example, Zhou and Notten, “Development of reliable lithium microference electrodes for long-term in situ studies of lithium-based battery systems,” J. Electrochem. Soc. 151(12) (2004) A2173-A2179 report the use of micrometer-scale wire as a lithium reference electrode. The micrometer size wire is sandwiched between the positive and negative electrode. This allows very short distance to the target electrodes to minimize the IR drop while avoiding significant distortion of current pathway due to the shielding effect. However, its use at high rate is questionable when the shielding effect becomes more pronounced. In addition, it can be impractical to implement micrometer-size reference electrodes.
In U.S. Patent App. Pub. No. 2011/0250478, Timmons and Verbrugge disclose the use of a cluster or array of reference electrode materials to monitor the state of charge of the positive and negative electrodes in a lithium-ion battery. An array of lithium-containing reference electrode materials is disposed on a common substrate. Each of the reference electrode materials can be a very small amount. Multiple reference electrode materials are used to determine the potential drifts.
In U.S. Pat. No. 8,163,410, Fulop et al. disclose the use of lithium, lithium titanium oxide and lithium iron phosphate as reference electrodes for battery state of charge and state of health monitoring. The locations of the reference electrodes include on the surface of the can or the endcap of the battery. A wire format reference electrode is disposed near the edge of the electrode stack or sandwiched in between the layers.
A practical approach is needed to improve battery diagnosis and battery management systems for the current state-of-the-art batteries (such as Li-ion batteries). The direct benefits include improving battery monitoring, enhancing battery safety, better understanding of battery aging diagnosis, and extending battery life. Direct measurements of the battery electrode potentials can greatly improve the battery safety and enhance the accuracy and reliability of battery managements.
In light of these and other shortcomings and needs in the art, improved battery structures are needed. Knowledge of battery health/life information as well as battery safety is economically critical in the marketplace. An improved battery structure and reference electrode design to measure the potentials of the positive and negative electrodes is desired, in order to facilitate battery state of charge and state of health monitoring.